bpf/selftests: Test fentry attachment to shadowed functions

Adds a new test that tries to attach a program to fentry of two
functions of the same name, one located in vmlinux and the other in
bpf_testmod.

To avoid conflicts with existing tests, a new function
"bpf_fentry_shadow_test" was created both in vmlinux and in bpf_testmod.

The previous commit fixed a bug which caused this test to fail. The
verifier would always use the vmlinux function's address as the target
trampoline address, hence trying to create two trampolines for a single
address, which is forbidden.

The test (similarly to other fentry/fexit tests) is not working on arm64
at the moment.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/5fe2f364190b6f79b085066ed7c5989c5bc475fa.1678432753.git.vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/bpf/test_run.c

@@ -560,6 +560,11 @@ long noinline bpf_kfunc_call_test4(signed char a, short b, int c, long d)
 	return (long)a + (long)b + (long)c + d;
 }
 
+int noinline bpf_fentry_shadow_test(int a)
+{
+	return a + 1;
+}
+
 struct prog_test_member1 {
 	int a;
 };

tools/testing/selftests/bpf/DENYLIST.aarch64

@@ -44,6 +44,7 @@ lookup_key                                       # test_lookup_key__attach unexp
 lru_bug                                          # lru_bug__attach unexpected error: -524 (errno 524)
 modify_return                                    # modify_return__attach failed unexpected error: -524 (errno 524)
 module_attach                                    # skel_attach skeleton attach failed: -524
+module_fentry_shadow                             # bpf_link_create unexpected bpf_link_create: actual -524 < expected 0
 mptcp/base                                       # run_test mptcp unexpected error: -524 (errno 524)
 netcnt                                           # packets unexpected packets: actual 10001 != expected 10000
 rcu_read_lock                                    # failed to attach: ERROR: strerror_r(-524)=22

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -268,6 +268,12 @@ static const struct btf_kfunc_id_set bpf_testmod_kfunc_set = {
 	.set   = &bpf_testmod_check_kfunc_ids,
 };
 
+noinline int bpf_fentry_shadow_test(int a)
+{
+	return a + 2;
+}
+EXPORT_SYMBOL_GPL(bpf_fentry_shadow_test);
+
 extern int bpf_fentry_test1(int a);
 
 static int bpf_testmod_init(void)

tools/testing/selftests/bpf/prog_tests/module_fentry_shadow.c

@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Red Hat */
+#include <test_progs.h>
+#include <bpf/btf.h>
+#include "bpf/libbpf_internal.h"
+#include "cgroup_helpers.h"
+
+static const char *module_name = "bpf_testmod";
+static const char *symbol_name = "bpf_fentry_shadow_test";
+
+static int get_bpf_testmod_btf_fd(void)
+{
+	struct bpf_btf_info info;
+	char name[64];
+	__u32 id = 0, len;
+	int err, fd;
+
+	while (true) {
+		err = bpf_btf_get_next_id(id, &id);
+		if (err) {
+			log_err("failed to iterate BTF objects");
+			return err;
+		}
+
+		fd = bpf_btf_get_fd_by_id(id);
+		if (fd < 0) {
+			if (errno == ENOENT)
+				continue; /* expected race: BTF was unloaded */
+			err = -errno;
+			log_err("failed to get FD for BTF object #%d", id);
+			return err;
+		}
+
+		len = sizeof(info);
+		memset(&info, 0, sizeof(info));
+		info.name = ptr_to_u64(name);
+		info.name_len = sizeof(name);
+
+		err = bpf_obj_get_info_by_fd(fd, &info, &len);
+		if (err) {
+			err = -errno;
+			log_err("failed to get info for BTF object #%d", id);
+			close(fd);
+			return err;
+		}
+
+		if (strcmp(name, module_name) == 0)
+			return fd;
+
+		close(fd);
+	}
+	return -ENOENT;
+}
+
+void test_module_fentry_shadow(void)
+{
+	struct btf *vmlinux_btf = NULL, *mod_btf = NULL;
+	int err, i;
+	int btf_fd[2] = {};
+	int prog_fd[2] = {};
+	int link_fd[2] = {};
+	__s32 btf_id[2] = {};
+
+	LIBBPF_OPTS(bpf_prog_load_opts, load_opts,
+		.expected_attach_type = BPF_TRACE_FENTRY,
+	);
+
+	const struct bpf_insn trace_program[] = {
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_EXIT_INSN(),
+	};
+
+	vmlinux_btf = btf__load_vmlinux_btf();
+	if (!ASSERT_OK_PTR(vmlinux_btf, "load_vmlinux_btf"))
+		return;
+
+	btf_fd[1] = get_bpf_testmod_btf_fd();
+	if (!ASSERT_GE(btf_fd[1], 0, "get_bpf_testmod_btf_fd"))
+		goto out;
+
+	mod_btf = btf_get_from_fd(btf_fd[1], vmlinux_btf);
+	if (!ASSERT_OK_PTR(mod_btf, "btf_get_from_fd"))
+		goto out;
+
+	btf_id[0] = btf__find_by_name_kind(vmlinux_btf, symbol_name, BTF_KIND_FUNC);
+	if (!ASSERT_GT(btf_id[0], 0, "btf_find_by_name"))
+		goto out;
+
+	btf_id[1] = btf__find_by_name_kind(mod_btf, symbol_name, BTF_KIND_FUNC);
+	if (!ASSERT_GT(btf_id[1], 0, "btf_find_by_name"))
+		goto out;
+
+	for (i = 0; i < 2; i++) {
+		load_opts.attach_btf_id = btf_id[i];
+		load_opts.attach_btf_obj_fd = btf_fd[i];
+		prog_fd[i] = bpf_prog_load(BPF_PROG_TYPE_TRACING, NULL, "GPL",
+					   trace_program,
+					   sizeof(trace_program) / sizeof(struct bpf_insn),
+					   &load_opts);
+		if (!ASSERT_GE(prog_fd[i], 0, "bpf_prog_load"))
+			goto out;
+
+		/* If the verifier incorrectly resolves addresses of the
+		 * shadowed functions and uses the same address for both the
+		 * vmlinux and the bpf_testmod functions, this will fail on
+		 * attempting to create two trampolines for the same address,
+		 * which is forbidden.
+		 */
+		link_fd[i] = bpf_link_create(prog_fd[i], 0, BPF_TRACE_FENTRY, NULL);
+		if (!ASSERT_GE(link_fd[i], 0, "bpf_link_create"))
+			goto out;
+	}
+
+	err = bpf_prog_test_run_opts(prog_fd[0], NULL);
+	ASSERT_OK(err, "running test");
+
+out:
+	btf__free(vmlinux_btf);
+	btf__free(mod_btf);
+	for (i = 0; i < 2; i++) {
+		if (btf_fd[i])
+			close(btf_fd[i]);
+		if (prog_fd[i] > 0)
+			close(prog_fd[i]);
+		if (link_fd[i] > 0)
+			close(link_fd[i]);
+	}
+}